The present invention relates to a method of regenerating sulfated metal carbonates or oxides particularly those of calcium that have been sulfated in the removal of sulfur dioxide from combustion exhaust gases.
In the fluidized bed combustion of coal, as well as the combustion of other sulfur-bearing fuels objectionable sulfur oxide gases are emitted with the exhaust. These oxides of sulfur are sorbed into alkaline earth metal or alkali metal carbonates or the corresponding oxides, produced by calcination within the combustion process.
Synthetic sorbents such as calcium oxide impregnated within a porous substrate of, for instance, alumina have been considered for removal of sulfur oxide gases. Such synthetic sorbents are described in copending U.S. patent application to Vogel et al. entitled "A Method Of Removing Sulfur Emissions From A Fluidized Bed Combustion Process", Ser. No. 684,419, filed May 7, 1976.
For environmental and economic reasons, a clear need exists for a regeneration process that will permit the cyclic use of the sorbent material and an efficient recovery of sulfur in an usable or storable form. Two regeneration processes are currently receiving considerable attention. The first is a two-step approach described by equations 1 and 2, EQU CaSO.sub.4 + 4H.sub.2 (4CO).fwdarw.CaS + 4H.sub.2 O(4CO.sub.2) (1) EQU caS + CO.sub.2 + H.sub.2 O.fwdarw.CaCO.sub.3 + H.sub.2 S (2)
regeneration based on these reactions can be carried out at moderate temperatures of about 800.degree. C. to 900.degree. C. for reaction (1) and about 500.degree. C. to 600.degree. C. for reaction (2). Elevated pressures of 8-10 atm. are required in order to obtain acceptable levels of regeneration. Even with these conditions only 50 to 60% completion in the regeneration of the sulfated calcium to calcium oxide has been obtained.
Another contemplated regeneration scheme is a one-step decomposition. EQU CaSO.sub.4 + H.sub.2 (CO).fwdarw.CaO + H.sub.2 O(CO.sub.2) + SO.sub.2 ( 3)
this reaction has been found to proceed to an acceptable 70% regeneration of the sulfated calcium and at atmospheric pressure but must be carried out at elevated temperatures of about 1100.degree. C. Process temperatures of such a high level are extremely undesirable due to the severe penalties resulting from increased corrosion and loss in material strength.
The reaction of the present development has been used previously as a method of producing SO.sub.2 in sulfuric acid processes. In such processes the reactants are provided in powdered form to achieve good mixture. However, it has not been seriously considered as a means of regenerating sulfated calcium oxides as it involves a solid-solid reaction which was thought to be difficult to carry out to a sufficient degree of completion within relatively large particles instead of blended powders. The reaction is as follows EQU 3CaSO.sub.4 + CaS .fwdarw. 4CaO + 4SO.sub.2 ( 4)
prior workers, having felt this reaction occurred to only a small extent, also believed the formation of calcium sulfide to be undesirable when carrying out the regeneration reaction given in equation (3) above, Combustion, Vol. 46/No. 7, "Combustion and Desulfurization of Coal in a Fluidized Bed of Limestone", page 10, January 1975; also see "Reduction of Atmospheric Pollution by the Application of Fluidized-Bed Combustion and Regeneration of Sulfur-Containing Additives", Annual Report July '71 - June '72, ANL/ES-CEN-1005, P. 43, Argonne National Laboratory.